Homework 8-solutions - el56 Homework 8 Walker (53970) 1...

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Unformatted text preview: el56 Homework 8 Walker (53970) 1 This print-out should have 37 questions. Multiple-choice questions may continue on the next column or page find all choices before answering. 001 1.0 points Consider the reaction CH 4 (g) + I 2 (g) CH 3 I(g) + HI(g) Bond energy tables give the following values: C H : 416 kJ/mol I I : 151 kJ/mol H I : 299 kJ/mol C I : 213 kJ/mol This reaction is 1. endothermic, but insufficient information given to say by how much. 2. endothermic by about 206 kJ/mol. 3. exothermic by about 206 kJ/mol. 4. exothermic by about 55 kJ/mol. 5. endothermic by about 55 kJ/mol. cor- rect Explanation: C H = 416 kJ/mol I I = 151 kJ/mol H I = 299 kJ/mol C I = 213 kJ/mol H rxn = summationdisplay BE rct- summationdisplay BE prod BE rct : 4 (C H) : 4 (416 kJ / mol) = 1664 kJ / mol 1 (I I) : 1 (151 kJ / mol) = 151 kJ / mol summationdisplay BE rct = 1815 kJ / mol BE prod : 3 (C H) : 3 (416 kJ / mol) = 1248 kJ / mol 1 (C I) : 1 (213 kJ / mol) = 213 kJ / mol 1 (H I) : 1 (299 kJ / mol) = 299 kJ / mol summationdisplay BE prod = 1760 kJ / mol H rxn = summationdisplay BE rct- summationdisplay BE prod = (1815- 1760) kJ / mol = +55 kJ / mol Therefore the reaction is endothermic by about 55 kJ/mol since the sign of H rxn is positive. 002 1.0 points The bond enthalpy in NO is 632 kJ / mol and that of each N O bond in NO 2 is 469 kJ / mol. Explain the difference in bond enthalpies be- tween the two molecules. 1. NO has a double bond while in NO 2 the bonds are equivalent due to resonance. cor- rect 2. Double bonds always have a larger bond enthalpy than single bonds. 3. The bond type does not explain the dif- ference. 4. NO has a single bond while in NO 2 the bonds are equivalent due to resonance. Explanation: The Lewis structures N b b b O b b b b for NO and N b O b b b b b b O b b b b N b O b b b b b b O b b b b for NO 2 show that the bond order in NO is the double bond and that in NO 2 is 1 . 5 the aver- age. The bond enthalpy of NO at 632 kJ / mol is close to the N O value listed in the table, whereas the N O bond enthalpy in NO 2 of 469 kJ / mol is slightly greater than the average of a N O and N O bond: 630 kJ + 210 kJ 2 = 420 kJ The extra stability is due to resonance sta- bilization. The bond enthalpies are the same for the two bonds, because the bonds are equivalent due to resonance. 003 1.0 points Consider the reaction of hydrogen and oxygen to produce water. el56 Homework 8 Walker (53970) 2 1500 1000 500- 500 Break 1 mol O=O bonds (+498 kJ) Form 4 mol H- O bonds (- 1868 kJ) Break 2 mol H- H bonds (+872 kJ) 2 H 2 + O 2 (reactants) Net energy change (- 498 kJ) 2 H 2 O (product) Energy(kJ) Break 1 mol of O O bonds: +498 kJ Break 2 mol of H H bonds: 2(+436 kJ) = +872 kJ Form 4 mol of H O bonds: 4(- 467 kJ) =- 1868 kJ Net energy change:- 498 kJ According to the figure, 1. the reaction of hydrogen and oxygen to produce water has a low activation energy....
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This note was uploaded on 05/05/2008 for the course CH 304k taught by Professor Walker during the Spring '08 term at University of Texas at Austin.

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Homework 8-solutions - el56 Homework 8 Walker (53970) 1...

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